Noise suppressor



Filed April 6, 1952' 5 Sheets-Sheet 1 INVENTOR JOHN KELLY JOHN SON XBY PW/AMJMM M ATTORNEYS 3 Sheets-Sheet 5 J.'K. JOHNSON NOISE SUPPRESSOR Filed April 6, 1932 March 28, 1939.

Q gm 9 8% g mw v Q \IQ\ V\ RN \I 8m v INVENTOR JQHN KELLY JOHNSON BY PM,

Ma a/4h,

ATTORNEYS mama .91 70,1

Patented Mar. 28, 1939 UNITED STATES PATENT OFFICE NOISE SUPPRESSOR Application April 6, 1932, Serial No. 603,490

Claims.

The present invention relates to signal transmission apparatus, and more particularly to a noise suppressor for automatically controlling the transmission through a signaling system to prevent the reproduction of disturbances, both channel and interchannel, whether occasioned by atmospheric conditions or produced within the system and tubes, whenever the noise intensity is greater than the signal intensity, as, for instance in a radio receiver, when tuning from station to station.

This is accomplished, in accordance with the present invention, by a noise-suppressor arrangement in which the transmission of signals through some portion of the system is controlled in such a manner that when the average signal intensity falls below a predetermined amount, the portion thus controlled will be rendered partially or completely inoperative and thus prevent transmission of the signals therethrough.

The features of this invention are preferably, though not necessarily, incorporated into radio receivers of the usual automatic volume control type, wherein the radio-frequency amplification is automatically controlled in accordance with the average signal intensity. Such a system is shown, for instance, in United States Letters Patent No. Re. 19,744, granted October 29, 1935, upon the application of Harold A. Wheeler.

With a receiver having the usual automatic volume control arrangement, when operating with the manual volume control so adjusted as to give the maximum volume, the sensitivity of the receiver is greatly increased when no signal is being received, as when tuning from station to station. This results in amplifying static and the effect of other atmospheric disturbances to a high degree, causing unpleasant sounds to be emitted from the loud-speaker.

Also, when such a receiver is not tuned to a signal channel and is therefore subject to interchannel disturbances, it is being operated so that the total amplification is maximum, and the radio-frequency amplifier tubes, While being op- 45 erated near their practical limit of amplification, cause the reproduction of disagreeable noises by the loud-speaker.

In view of the fact that in the usual automatic volume controlled receiver the sensitivity of the set is decreased in proportion to the signal intensity, it is necessary to arrange the automatic volume control to have a time delay action to prevent reduction of the lower notes of the audio-frequency signal. This time delay action may not permit the reduction of the amplification of the receiver as the tuning crosses the frequency allocations of undesired signals in tuning rapidly through several stations, and may thus cause the loud-speaker to produce a series of loud, unpleasant noises.

It is an object of the present invention to eliminate the above-noted disadvantages of the automatic volume control system, to prevent the reproduction of noise, whether produced by atmospheric disturbances or by the actions of the tubes themselves, and to prevent unpleasant sounds as the receiver is rapidly tuned across the frequency allocations of several stations, as in tuning to a desired signal.

This and further objects will become apparent from the following specification taken in connection with the accompanying drawings.

The invention is preferably carried out by translating or amplifying the signal voltage at carrier frequency; amplifying the signal voltage at lower frequency than the carrier frequency when a carrier frequency is being received; suppressing the amplification at the lower frequency when there is little or no carrier signal voltage being received; translating only the resulting and desired energy into sound waves; and controlling the signal voltage level below which the amplification at the lower frequency is suppressed. More specifically, the manner by which the amplification at the lower frequency is suppressed is by deriving from the signal voltage a uni-directional potential which is representative of, that is, varies in accordance with the amplitude of the carrier frequency or radio frequency, and applying a biasing voltage derived from the uni-directional, or direct current, potential to a control element of the translating system.

In accordance with a feature of this invention, a noise-suppressor tube, as it will be called hereinafter, is provided. The input circuit of the tube is supplied with a voltage representative of the direct-current component of the detected signal. This tube is so operated that it serves to reverse the fluctuations of input voltage and impress the amplified and reversed direct-current voltage fluctuations upon a control circuit of some tube of the receiver, preferably a tube following the detector, such, for instance, as the second detector tube in a superheterodyne radio receiver, or an audio-frequency amplifier tube. Hereinafter the tube controlled by the noisesuppressor tube will be referred to as the controlled tube.

The characteristics of the suppressor and controlled tubes are so chosen that a substantial change in the voltage of the output of the suppressor tube is obtained by a relatively small variation in the received signal intensity to vary the bias of the controlled tube between a value which will prevent the passage of signal voltage therethrough, or will bias it to cut-off, and a value which will permit normal gain in the system. Thus, when no input voltage due to received signals is impressed upon the suppressor tube the controlled tube is biased to cut-off, thereby preventing signal voltage impressed upon the input thereof from becoming effective in the output circuit. The cut-off bias is ordinarily negative, that is, less positive than is required for normal transmission; hence, when operated in the cut-off condition, the entire controlled system offers in effect a high impedance to the transmission of signal voltages therethrough. Conversely, when an appreciable signal voltage is present, the cut-off bias of the controlled tube is altered, and hence the impedance of the controlled system is in effect lowered, thus permitting the passage of signals therethrough.

It can be seen that whereas (when receiving no signal) the radio-frequency amplifiers of the usual automatic volume control receiver will have the greatest sensitivity, due to the automatic volume control feature, under the same conditions, with a receiver embodying the present invention, the controlled tube is biased to cut-off, and therefore no sound impulses are reproduced by the loud-speaker. Whenever a signal of greater than a predetermined intensity is received, the radio-frequency gain is decreased, and simultaneously the bias on the controlled tube is thereby adjusted to the proper value to allow normal amplification.

The arrangement constituting the present invention prevents the production of unpleasant noises when tuning rapidly through several carrier signal frequency channels. This is accomplished by introducing a time delay action into the noise-suppressor circuits so that the cut-off bias of the controlled tube is not removed rapidly enough to let each signal through to produce sounds in the loud-speaker.

A feature of the invention is the provision of variable means for determining the carrier frequency signal strength required to decrease the internal impedance of the controlled tube to the point where it becomes inoperative to transmit signals, that is, for predetermining the cut-off level. This is preferably effected by means for controlling the characteristics of the suppressor tube circuit so that any desired input voltage level may be required to cause the controlled tube to operate. This provides a control for the cut-off level, which term will be employed hereinafter to mean the minimum level at which the suppressor tube will operate to permit reproduction of signals. Thus if the level of noise is high, the cut-off-level control may be operated to raise the cut-off level so that an input voltage greater than the general noise level will be required before the controlled tube will operate. Conversely, if there is a very low noise level, the cut-oif-level control may be manually operated to adjust the characteristics of the suppressor tube circuit so that a small input signal voltage will be reproduced.

In order to permit an adjustment of the cutoff level which will not have to be altered while tuning through the frequency band of the 112-.

ceiver, uniform-gain radio-frequency circuits are preferably employed.

Having. thus briefly described the present invention, attention is invited to the accompanying drawings, in which:

Fig. 1 is a simplified schematic diagram of a signal translating system incorporating the invention;

Figs. 2a, 2b, 2c, 2d, 22, and 2] are diagrams illustrating the operation of the circuit of Fig. 1;

Fig. 3 is a circuit diagram of a more practical embodiment of the present invention, and

Fig. 4 is an overload-characteristic curve of a radio receiver for illustrating the operation of a receiver embodying this invention.

The circuit of Fig. 1 includes a radio-frequency, or carrier-frequency, signaling system comprising an amplifier II which is adapted to amplify the incoming signal voltage at carrier frequency and impress the amplified signal upon the input terminals of the vacuum tube diode detector or rectifier I2 by means of a coupling transformer having primary and secondary windings 20 and 2|, respectively. The detected signal is then amplified by the audio-frequency amplifier 13, which is the controlled tube in the embodiment shown, and the amplification of the detected signal is controlled, in accordance with the present invention, by means of a noisesuppressor tube 14, the operation of which will be explained in connection with the following more detailed description of the circuit arrangement. Each of the devices ll, 12, I3 and I4 is an electron discharge valve having a space path and a plurality of electrodes. commonly called vacuum tubes. Tubes ll, [3 and I4 each have a cathode, an anode and a control electrode, as shown, while tube 12, although shown with a cathode, anode and a grid, effectively has only an anode and a cathode for operation as a diode, as will more fully appear hereinafter.

The input of the radio-frequency amplifier tube ll includes a tuned circuit l5 for tuning to carrier-frequency channels throughout a range of channel frequencies, and the output of this amplifier includes the output inductance 20, which is inductively related to the input inductance of the tuned input circuit 21 of the detector l2. resistor 22, by-passed by the radio-frequency bypass condenser 23. The resistor 22 and the condenser 23 are so proportioned as to cause the tube l2, which is connected as a diode, to operate as a peak detector, as has been described in the application of Harold A. Wheeler mentioned above. There is developed in resistor 22 a uni-directional output component of the detected signal, which varies upon variation of the signal impressed upon the input of the detector.

One side of the resistor 22 is connected through resistor 24 to the tuned circuit l5 of the radio-frequency amplifier II. The output circuit of the amplifier II is completed through the high-potential source 25, and a biasing battery 30 is provided to give the grid of the tube the proper bias, as will be explained hereinafter. The alternating current detected signals are impressed upon the input circuit of amplifier [3 through the coupling condenser 3| and resistor 32, the voltage drop across a portion of which is impressed between the control grid and thecathode of the amplifier tube l3. The amplifier I3 is provided with a grid biasing source 35 and with an output circuit comprising output These valves are p The detector output circuit includes the inductance 36 and high-potential source 31. The inductance 36 is inductively coupled to the inductance 38, which, by means of the terminals 39, may be employed to supply a loud-speaker or the input of further vacuum tube amplifiers. The voltage fluctuations existing across the resistance 22 are impressed directly upon the grid of the noise-suppressor tube I4, the output circuit of which includes the portion of the resistor 32 below the tap point 34. The uni-directional voltage across the resistor 32 is therefore responsive to the uni-directional voltage at the output of the detector, or rectifier, I 2, and is amplified by the tube I4; the resistor 32 may therefore be considered to be in a circuit associated with the detector. There is a direct connection from the point 34 of the resistive impedance 32 to the control element of the controlled tube I3. The

tube I4 amplifies the uni-directional, i. e., the

direct-current component of the detected signal in such a manner as to change the direction of fiuctuation thereof. Potential source 33 is provided for supplying the plate-potential for the plate of tube I4.

The noise-suppressor tube should preferably have a high amplification factor and should have a sharp cut-off in its grid-voltage platecurrent characteristic curve in order that a relatively small change in grid voltage will make a very decided change in the plate current. A type 221 tube is satisfactory for this purpose.

The controlled tube, in this case the audio-frequency amplifier I3, should have a sufilciently high amplification factor that a small increase in negative grid bias will bias the tube to cut-off and will stop substantially all plate current. A tube which has been found satisfactory for this purpose is the type 221 tube.

The condensers by which circuits I and 2I are tuned may be electrically similar and connected to be simultaneously operated as indicated by the dotted line connecting them in Fig. 1.

In operation, the high-frequency amplifier tube I I is biased by means of the potential source 33 so that it will have maximum sensitivity with no signal present. The presence of a high average current through the resistor 22 will cause the grid of tube I I to become more negative relative to the cathode thereof and consequently reduce the amplification of the high-frequency amplifier.

With no radio-frequency signal input to the detector I2, the grid of the noise-suppressor tube I4 will have substantially the same potential as the cathode thereof, and a maximum plate current will flow through the lower portion of the resistor 32. This will cause the point 34 and the grid of the audio-frequency amplifier I3 to be biased so much more negative than the normal bias voltage produced by the bias source 35 that it will be biased to cut-off or past, that is, to a negative value sufiicient to prevent the tube from transmitting signals; and no amplification or transmission through this tube will take place when the alternating voltage intensity at the point 34, and hence at the control element of tube I3, is relatively small. It is observed, then, that resistor 32 is an impedance across which is developed a uni-directional voltage responsive to, or derived from the output of the detector I2 which varies upon variation of the strength of the impressed signal. It is thus apparent that when the grid of audio-amplifier I3 is biased more negatively in response to a smaller received signal intensity, its mutual conductance, and

hence the amplification, decreases, that is to say, its effective internal impedance increases.

Thus, in the condition of no radio-frequency signal, although the amplifier tube II is operating at maximum efficiency and the receiver is in its most sensitive condition, as there is no, or very little, average flow of rectified current through the resistor 22, the bias of the tube I4 will be such that a substantial current will be passing through the resistor 32, and the tube, or valve, I3 of the audio-frequency amplifier system will therefore be inoperative to transmit stray voltage fluctuations which pass through the detector and are impressed upon the grid of the tube through the coupling condenser 3|. If, on the other hand, an alternating current voltage signal is impressed upon the detector tube through the amplifier II, the average current through the resistor 22, and therefore the average negative voltage at the low end thereof, will increase and the plate current of the noise-suppressor tube I4 will be decreased, thus increasing the voltage at the point 34 with respect to ground, that is, making it less negative with respect to the cathode and permitting the tube I3 to act as an audio-frequency amplifier of the alternating current voltage signals impressed thereon from the detector through the coupling condenser 3I. Thus when little or no signal strength is being received, the control electrode of tube I3 is greatly negative but becomes less negative (i. e., more positive) when the signal strength increases.

It is to be understood that, due to the automatic volume control of the radio-frequency amplifier II, if there is any substantial signal present at all, the average current through the resistor 22 will be sufiiciently high to cut off the plate current in tube I4 and will leave the negative biasing voltage of the point 34 substantially constant over the range of received signal intensities required for normal operation. If, on the other hand, no signal is present, there will be substantially no current flowing through resistor 22, and the point 34 will have a potential which will cause sufficient current in resistor 32 to bias the amplifier I3 to out-off. It is observed that the polarity of point 34 is that which causes the control element, or grid, of tube I3 to become less negative (or in other words, more positive) when the alternating current voltage at tube I2, and hence at tube I3, increases, whereby the internal impedance of tube I3 is decreased as the alternating current voltage increases.

The specific details of the method shown for obtaining the automatic volume control effect, i. e., automatically controlling the amplification of the carrier-frequency signal voltage, constitute no part of the present invention, it being understood that any appropriate method may be utilized by which the amplification of the radio-frequency amplifiers and/or thetranslation gain of the first detector and intermediatefrequency amplifier of a superheterodyne are controlled. The noise-suppressor effect may be obtained by controlling the amplification of any tube or tubes following that which supplies the suppressor tube, such, for instance, as an intermediate-frequency amplifier, second detector, or first or second audio-frequency amplifier of a superheterodyne receiver.

For the purpose of explaining the function of the circuit just described, attention is now invited to Figs. 2a through 2 inclusive. In Fig. 2a the ordinates represent the radio-frequency voltage in the input of the amplifier N (Fig. 1), whereas the abscissas represent the frequency to which the input circuit I5 is tuned. Curve 4011 represents the voltage impressed upon the grid of tube II as the input circuit is tuned through the frequency of transmission of a signal which is received with medium intensity. Curves Ma and 42d similarly represent the grid voltages of tube I as the circuit |5 is tuned through the frequencies 4| and 42 of signals being received with low and high intensity, respectively. Curve 43a represents the voltages impressed upon the tube due to stray atmospheric disturbances and due to tube noises, etc., in the preceding amplifier stages.

Fig. 2b shows a curve 4411, which represents the radio-frequency amplification in the tube II as the circuits l5 and 2| are tuned through the range of frequencies covered by Fig. 2a. The portions 431), which show substantially uniform radio-frequency amplification, indicate the amount of amplification of the ground noise, etc., shown by the curve 43a in Fig. 2a. The portions of the curve 44b designated 4012, MD, and 4212 indicate the decrease in radio-frequency amplification as the circuits I5 and 2| are tuned through the signal frequencies 40, 4|, and. 42, respectively.

In Fig. 2c the curve 440 represents the peak audio-frequency input to the tube I3. The portions 430 are the peak input voltages resulting from the amplification of the ground noise indicated by curve 43a. The portions 400, Mo and 420 are the peak voltages produced as the stations 4|], 4| and 42 are received. The dotted curve 41 represents the average voltages existing across the resistor 22. Curve 41' indicates the average voltage representative of a much higher ground noise level, as will be explained hereinafter.

As explained above, whenever the Voltage across the resistor 22 increases above a. certain predetermined value, indicated by a decrease in the radio-frequency amplification at point in Fig. 2b, assuming the circuits to be tuned from left to right, the tube M will be biased to cut-off, which will remove the cut-off bias potential of the grid of tube l3. This results, then, in an audio-frequency amplification by the tube l3, which'is indicated in Fig. 2d, in which the portions 40d, Md and 42d represent the audiofrequency amplification existing as the receiver circuits are tuned through the frequency of the signals 40, 4| and 42 respectively. It will be noted that the sides of the curves 42d, as represented at 45d and 46d respectively, are decidedly abrupt. This abrupt change in amplification takes place as the average detector voltage across the resistor 22 passes points which may be adjusted as desired and which are indicated at 45c and 460 respectively. Between these points on the curve 41, indicating the average voltage across the resistor 22, a signal of relatively uniform intensity is being impressed upon the detector circuit.

In Fig. 2c, the combined amplification of the tubes H and I3 is indicated. The portions. of the curve 40c, for instance, are represented as being composed of the abrupt rise in audio amplification, 40e, the dip in radio-frequency amplification, 40b, between the points 45c and 466 caused by the decrease in radio-frequency amplification as indicated in Fig. 2b, and the decrease in audio amplification indicated, 40"e.

The audio-frequency output of the receiver is indicated by curves 40f, 4| 1 and 42f, respectively, of Fig. 2f, which curves represent the overall output when tuning through the frequency of signals 4|], 4| and 42, respectively.

The cut-01f of the audio-frequency tube is determined by the average voltage across the resistor 22, and therefore if the noise is heavy, depending upon the characteristics of the noisesuppressor-tube circuits, sufiicient voltage will be built up across the condenser 23 and the resistor 22 to result in a decrease in the bias of the audio-frequency amplifier l3, which will permit the noise to pass through although no station is being received. As above noted, the point at which the tube I4 is biased to prevent its drawing plate current, and thus remove the blocking bias on the tube l3, may be adjusted manually to suit the particular conditions, which will result in cutting out more or less of the noise. A convenient way of accomplishing this adjustment is by using a screen-grid tube for the noise-suppressor tube I4, and providing means for varying the screen voltage. By this arrangement the amplification of the uni-directional component of the detector may be varied, and the cut-off may be adjusted so that the required voltage across the resistor 22 to start audio-frequency amplification will not be obtained in the absence of a signal.

A circuit for accomplishing this result i. e., for providing adjustable means for selecting the cutofi level above which the controlled tube is enabled to transmit signals, is shown in Fig. 3, which circuit is quite similar to that shown in Fig. 1, similar parts being indicated by the same reference characters. This figure differs from the above-described figure principally in the provision of a unitary power supply source, the plate and grid voltage portions of which only are shown. In this circuit, a series of resistors is connected between the high-potential inputs, the negative terminal of which is represented at 49 and the positive terminal of which is represented at 50. The ground of the set is connected to an intermediate point 5|. The bias voltage between the grid and cathode of tube H is determined by the portion of the resistor 22 between terminal 22' and the right end thereof, resistor 9 and the resistance between terminals 49 and 5| When no signal is being received, the current flowing through the resistance between terminals 49 and 5! will provide the initial grid bias. The bias required for the automatic volume control feature is produced by the detected current flowing through the resistor 22. The grid bias of the tube M is provided by means of the entire resistor 22. A resistor l! is included in the grid lead of the tube M to prevent impressing radiofrequency signals on the grid of tube l4, and a by-pass condenser I8 is also provided for this same purpose. A radio-frequency by-pass condenser is also connected between the lower end of the tuned circuit l5 and the cathode of tube II. The screen potential of the tube |4 may be adjusted, as desired, by means of the variable contact 52 on the resistor between terminals 5| and 53. The contact between the leads from the plate and grid of tubes l4 and I3, respectively, is made variable, as indicated at 34, by means of which the volume output of the receiver may be controlled. This variable contact is a circuit connection which variably controls the output of the tube M to determine the impressed signal strength required to substantially decrease the internal impedance of the controlled tube. The plate circuits of tubes l4 and I3 include resistors 28 and 48, respectively, for the purpose of properly adjusting the potentials of these elements. Appropriate radio-frequency bypass condensers 21 and 29 are also provided across these resistors.

The tap 22' on the resistor 22 may also be connected to preceding high-frequency amplifier tubes, as desired, to permit automatic control of the amplification of such tubes. The circuit of Fig. 3 is primarily intended to include the intermediate-frequency amplifier and second detector of a superheterodyne radio receiver, in which case the circuits l5 and 2! would be tuned to intermediate frequency, and additional radio-frequency amplifier tubes and first detector tubes would be provided, the grid bias potentials of which would be automatically controlled by connection to the tap 22" of the resistor 22.

The high amplification factor of the screengrid noise-suppressor tube permits a small change in the average voltage across the resistor 22 to give a large change in the plate current and therefore a large alteration in the bias of the audio amplifier. Thus, the required change in voltage across the resistor 22 being very slight, the audio-frequency amplifier is effectively turned on and off as the set is slowly tuned through a signal channel.

The operation of the noise-level cut-oil control, i. e., the means for adjustably determining the alternating current voltage intensity above which the internal impedance of the controlled tube is substantially decreased, which is the most important particular in which this figure differs from the circuit of Fig. 1, will now be described. Referring to Fig. 2c, assume that with the audio cut-01f working at a sensitivity of 0.3 volt as indicated at 450 the noise level is increased above 0.3 to 41. Then obviously the noise-suppressor tube would be biased to cut-off and the audio amplifier would operate. It is then desirable to decrease the sensitivity of the cut-off control to, say, 0.6 volt, as indicated at 450. To do this, the screen-grid voltage may be manually increased 2.4 volts by operation of the variable contact 52, which, assuming a gridscreen amplification factor of 8, will then necessitate that the grid be biased 0.3 volt more negative to operate the cut-off of the suppressor tube and cut in the audio amplifier. Then, although the average voltage existing across the resistor 22 is that shown by curve 41, ground noise will be completely suppressed while tuning between signals.

In further explanation of the operation of the cut-off level control, attention is now invited to Fig. 4, which shows an overload characteristic curve of an automatic volume control radio receiver. The ordinates are output volts from the audio amplifier and the abscissas represent signal input to the antenna, in micro-volts. The curve 60 gives the overall characteristic of a receiver operating in its most sensitive condition. This curve is taken with the noise output filtered out. The curve 60' is that portion of the output due to internal noise which was filtered out in curve 60. The curve 653 gives the total voltage output due to signal and internal noise plotted against input signal voltage.

The curve 6! shows the output of the audio amplifier when the set is operated in a less sensitive condition, as would be the case, for instance, With a local-distance switch in the local position. As indicated, the sensitivity of the receiver is then so reduced that there is no noise component in the output, attributable to internal noise.

The curve 6lll60"62 shows the output of a receiver employing a noise suppressor, constructed in accordance with the present invention, with the cut-ofi-level control set, say, for the lowest practical cut-01f level. This level has been, in this instance, selected as that at Which the result will give an audio output which is half noise and half signal, assuming the most ideal atmospheric condition of absolute freedom from atmospheric disturbances. This corresponds to the condition resulting in the curve l! of Fig. 20, where the distance between the base line and the curve between signals represents the audio input resulting from internal noise. This condition gives a sensitivity of about 4 micro-volts. Yet whenever the signal input falls below point 63, corresponding to about 3 micro-volts, the noise suppressor will operate and the controlled tube will allow no signal output.

If now the noise input level is increased to, say, micro-volts, indicated at point 64, giving an audio-frequency input as indicated by curve 41' in Fig. 2c, the cut-ofi level can be raised to point 65 by adjustment of the cut-off level control; and whenever a signal greater than, say, 30 microvolts is received, resulting in an audio-frequency input, indicated at 450 in Fig. 2c, the audio-frequency output as indicated by curve 6066 will be obtained. Whenever the signal falls below 30 micro-volts, the audio amplifier will be inoperative to transmit signals to the loud-speaker. Thus, if a 100 microvolt signal is being received, the output to the loud-speaker will be 202 volts, as indicated at 61. On the other hand, with a simple local-distance switch, under the same conditions, the output for a similar signal would be less than 50 volts, as indicated at point 68.

Similarly, if the noise level is greater, the cutoff level control may be operated to require a signal of 100 micro-volts, indicated at 69, to operate the suppressor tube to cause operation of the reproducing apparatus. The receiver would then operate as indicated by curve 6010.

The manual Volume control is a common arrangement in which any desired portion of voltage fiuctuation produced across a resistor included in the output circuit of a detector may be utilized to transmit the signals to the audio-frequency amplifiers. The connection from the plate of the noise-suppressor tube M to the resistor 32 may be at any desired point, such, for instance,

as at the top thereof, although the arrangement shown has been found quite convenient due to the fact that the portion of the resistor 32 included in the grid circuit of the tube l3 and plate circuit of tube l4 increases as the control is set to higher output volume, thus increasing the biasing cut-off voltage on the grid of tube l3 as increased suppression is needed.

In view of the fact that if the input gain is not uniform the sound level would rise and fall as the receiver were tuned and it could therefore not be eificiently operated with one setting of the cutoff-level control, it is, as above noted, desirable that the input circuits of a receiver embodying this noise suppression invention be of the uniform gain type. Otherwise the cut-ofi level control would have to be varied as the receiver were tuned through its range.

The operation of the circuit of Fig. 3 is in all other respects similar to that of Fig. 1, which has been described in full above.

The usual cathode heating circuits are provided, and any appropriate tubes may be substituted for those shown in both Fig. '1 and Fig. 3, the specific details of none of these elements constituting any part of the presentinvention'.

*What 'is claimed is:

1. In a radio receiver, means for preventing the reproduction of undesired sounds, which comprises a detector and an audio-frequency amplifier tube including input and output electrodes, a resistor, a connection between one end or said resistor, and the detector output, said connection including a coupling condenser whereby voltages of the audio-frequency component existing in the output of said detector are impressed upon said resistor, means for impressing any desired amount of the audio-frequency voltage existing across said resistor upon the input electrodes of said audio-frequency amplifier tube, a noise-suppressor tube having input, output, and screengrid electrodes, the input electrodes of said noisesuppressor tube being connected to the output of said detector, and said output electrodes being connected across said resistor, whereby said noisesuppressor tube controls the amplification of said audio amplifier, and means for varying the voltage 'of said screen-grid electrode of said suppressor tube and thereby regulating the level of cut-ofi of said tube.

2. In a radio receiver including a detector, in which the average output voltage of the detector is adapted to control the carrier-frequency amplification, the combination of an audio-frequency amplifier including input and output electrodes, a resistor connected between said input electrodes, a connection for impressing the audio-frequency component of the detected voltage across said resistor, a noise-suppressor tube having input, output, and screen-grid electrodes, means for impressing the direct-current component of the detector voltage across the input electrodes of said noise-suppressor tube, the output electrodes of said noise-suppressor tube being connected across said resistor, and means for varying the screen-grid voltage of said noise-suppressor tube and thereby regulating the level of cut-off of said tube.

3. In a radio receiver tunable over a frequency range, means for suppressing interchannel disturbances and voltages below a cut-off level, said means comprising a first tube having an input electrode, a screen and an anode, a second tube having a control electrode connected to said anode, said control electrode having impressed thereon a bias voltage which is more'negative than that which permits said second tube to transmit signals, means for deriving from received signals a unidirectional voltage which varies as the strength of said received signals, means for applying said unidirectional Voltage to the input of said first tube in the polarity which causes the voltage in the anode circuit of said first tube to render the bias of said second tube less negative, whereby signalvoltages are then transmitted through said second tube, means for applying a voltage to said screen, and means for varying said screen voltage for selecting the cutof! level above which said second tube is enabled to transmit signals.

4. In a carrier-frequency signaling system tunable over a range of carrier-frequency channels, an arrangement for suppressing interchannel disturbances and voltages below a predeterincluding at least one control electrode, a second tube, the bias of said last-named control elec trode and the polarity of said derived voltage applied thereto being such that the bias of said second tube is raised above cutofi only upon the reception of signals above said predetermined value, and means for applying an independently adjustable voltage to a control electrode of said tube to adjust said cut-off level.

5. In a carrier-frequency signaling system tunable over a range of carrier-frequency channels and including a detector and an audio-frequency amplifier, said amplifier being normally biased to cut-off, an arrangement for suppressing interchannel disturbances and voltages below a predetermined value comprising a screen grid noise suppressor tube coupled to said detector, an impedance in the output circuit of said tube for modifying the bias of said amplifier, the bias of said noise suppressor tube and the polarity of the voltage applied thereto from said detector being such that the bias of said amplifier is raised above cutoff only upon the reception of signals above said predetermined value, means for applying an adjustable voltage to the screen grid of said noise suppressor tube to adjust said cut-off level.

6. In a carrier-frequency signal receiver, a signal-translating tube including a control element, means for impressing across the input of said tube an alternating voltage derived from the car rier frequency input of said receiver and including adjustable impedance means for adjusting the portion of the derived alternating voltage applied to the input of said tube, and means including said impedance means for developing a bias voltage continuously variable in accordance with the amplitude of the carrier input to the receiver and for applying said bias voltage to the input of said tube, said tube being normally so biased and said bias voltage being applied thereto with such polarity that said tube is eifective to translate signals only upon the reception of signals above a predetermined amplitude, adjustment of said impedance means being effective to adjust the output of said tube and simultaneously the predetermined signal amplitude which the system is efiective to receive.

'7. In a carrier-frequency signal receiver, a carrier-frequency amplifier, means for controlling the amplification of said amplifier inversely in accordance with the carrier-frequency input thereto for maintaining the output of said amplifier within a narrow range for a wide range of received signals, a signal-translating tube including a control element, means for impressing across the input of said tube an alternating volt-' age derived from the output of said amplifier and including adjustable impedance means for adjusting the portion of the derived alternating voltage applied to the input of said tube, and means including said impedance means for developing a bias voltage continuously variable in accordance with the amplitude of the carrier input to the receiver and for applying said bias voltage to the input of said tube, said tube being normally so biased and said bias voltage being applied thereto with such polarity that said tube is effective to translate signals only upon the reception of signals above a predetermined amplitude, adjustment of said impedance means being effective to adjust the output of said tube and simultaneously the predetermined signal amplitude which the system is effective to receive.

8. A carrier-frequency signal receiver including, in cascade, a detector tube and an audiofrequency amplifier tube normally biased to cutoff, the connections between said detector tube and said amplifier tube including an adjustable vol ume control impedance, and means for developing in said impedance means a unidirectional bias voltage which varies continuously in accordancewith the amplitude of the carrier input to the receiver, said unidirectional potential being applied to said amplifier with such polarity that the bias thereof is raised above cutoff only upon the reception of signals above a predetermined amplitude, adjustment of said impedance means being effective to adjust simultaneously and in the same sense the volume output of the receiver and the cut-off level of said amplifier.

9. In an amplifier for radio signals, a radiofrequency amplifier tube, a detector connected to the output of said amplifier, a resistor shunted by a condenser associated therewith, whereby there is produced a rectified voltage having a direct-current component representative of the average signal current, connections from said resistor to the input of said radio-frequency amplifier, whereby the bias of said amplifier is controlled by the voltage across said resistor, an audio-frequency amplifier including input and output electrodes, a second resistor connected between said input electrodes, a connection between the grid end of said second resistor and said detector, whereby audio-frequency signals are impressed upon the grid of said amplifier, a noisesuppressor tube having input and output electrodes, said input electrodes being connected across said first-mentioned resistor, and said output electrodes being connected across said second-mentioned resistor, whereby when a high average voltage exists across said first-mentioned resistor said noise-suppressor tube is so biased that there will be no output current, and when said voltage falls below a predetermined amount said noise-suppressor tube will be sobiased that a current will flow in the output circuit thereof, which current, fiowing through the secondmentioned resistor, will bias the input circuit of said audio-frequency amplifier tube to cut-off.

10. Apparatus for amplifying radio signals which comprises a high-frequency amplifying circuit, an audi-frequency amplifying circuit, a detector circuit interposed between said amplifying circuits, means associated with the output of said detector circuit for controlling the amplification of said high-frequency amplifying circuit, said means comprising a resistor across which is imposed detected currents, a direct-current amplifier circuit adapted to inversely amplify the average voltage fluctuations imposed across said resistor, said circuit including a tube having control grid, screen grid, anode, and cathode, and means in the output of said directcurrent amplifier comprising a second resistor through which the plate current of said amplifier is adapted to flow, whereby when the direct-current component of the detection voltage is large, no plate current will flow in the plate circuit of said direct-current amplifier, and a connection between said second resistor and the audio-frequency amplifying circuit, whereby when plate current flows said audio amplifier is biased to cutoff, and the reproduction of noise may be eliminated when tuning said amplifier from one frequency setting to another.

JOHN KELLY JOHNSON. 

